The present invention relates generally to semiconductor devices, such as charge-coupled devices (CCDs"), in which charge representative of an input signal is collected and manipulated. More specifically, the invention relates to techniques for extending the dynamic range of such devices.
Charge-coupled devices, bucket brigade devices and other semiconductor devices in which charge is manipulated are well known. Such devices are used as shift registers, as memory cells, in analog signal processing as filters, and as charge-coupled imaging devices, both for line scan and area imaging purposes. The operation of these devices is similar in that depletion regions are created in a semiconductor substrate by application of an electric field, which regions are energetically favorable to the accumulation of charges. Accumulated packets of charge are then manipulated in these regions according to the needs of the particular application.
In known charge-coupled devices an initial charge may be injected into such a region electrically from circuit associated elements, such as in memory cell devices, or the charge may be generated within the region in response to incident radiant energy (i.e., visible light or infrared radiation), such as in imaging devices. In a CCD imaging device, charges are generated in a region of the semiconductor substrate, generally referred to as the photosite, which is sensitive to the incident radiation. The charges are allowed to accumulate, in a region referred to as the photogate, for a period of time generally referred to as the integration time. The amount of charge accumulated in the photogate region over an integration period provides a measure of the average radiant energy impinging on the photosite during the integration period. After each integration period the accumulated charge is transferred to a temporary storage region of the semiconductor substrate, typically forming a part of a shift register, where the charge is then passed to various circuit elements to be detected, amplified, and/or otherwise processed to provide an output signal representative of the incident light intensity.
A limitation on known charge-coupled devices is the relationship between the amount of charge which may be accumulated in the photogate region and the charged-handling capability required of all of the peripheral circuitry, such as the temporary storage location to which the photogate charge is transferred in the shift register. Increasing the size of the photogate allows more charge to accumulate over a given period and provides greater overall dynamic range for the device; however, such an increase in size also places a correspondingly higher requirement on the charge-handling capability of the shift register or other circuitry. Thus, the size of charge-coupled devices sufficient to provide wide dynamic range is undesirably increased by the extra charge-handling capability required. The increase in size of the shift registers and associated peripheral circuitry in turn increases the size of the dies used in manufacturing, which lowers the yield of functioning devices and increases the price for each device. Viewed alternatively, the increased size of the peripheral charge-handling circuitry decreases the chip area available for photosensing, and thereby lowers the dynamic range of the device.
One known approach for handling the occasional overcapacity of the photogate region was developed in connection with the phenomenon known as "blooming." The temporary storage region can hold a limited quantity of charge and/or can only transfer a limited quantity of charge at a time out of the region. If the quantity of charge accumulating in the photogate region exceeds the capacity of the associated temporary storage region, then when the charge is transferred from the photogate region, the temporary storage region exceeds its saturation limit and the excess charge spreads to neighboring regions associated with other light-sensing elements. This spreading or smearing of the charge is known as blooming and leads to a local loss of image resolution and diminishes the effective dynamic range of the device.
To counteract the blooming effect, an additional structure is provided on the CCD chip to handle excess charge. An antiblooming structure comprising a "sink" region and appropriate electrodes and gates is added adjacent to the photogate to receive the excess charge. In one approach charge is allowed to accumulate in the photogate for an integration time which is only a predetermined fraction of the exposure time. This is accomplished by opening a gate between the photogate and the sink for a fraction of the exposure period such that all the charge generated in the photosite when the gate is open is diverted to the sink. The remainder of the exposure period then provides the integration period. Only the fraction of the generated charge accumulated during the integration period is transferred to the temporary region in the shift register. In a further refinement an integration control gate may be biased such that even during the integration period accumulated charge exceeding the saturation level of the associated temporary storage region will be diverted to the sink. These techniques reduce the occurrence of blooming and provide improved dynamic range.